The most pertinent prior art of which applicants are aware is an article that appears in the IBM Technical Disclosure Bulletin, Vol. 23, April 1981, at pp. 5167-5168. This article describes a write driver circuit for use in a magnetic storage system that comprises four bipolar transistors. These transistors are arranged as upper and lower pairs in an "H" configuration to provide a bidirectional write current through a thin film inductive write head. To ensure fast switching, (i) saturation of the active one of the transducers of each pair is prevented by providing a sufficiently high supply voltage, and (ii) a low impedance path is provided to discharge the parasitic base capacitances of these active transistors upon turn off, so they will respond more rapidly when turned on. A write current source is interposed in series with a current switch in this H-configured write driver circuit.
While not a problem when supply voltage is high, there is a problem when supply voltages are as low as three volts because with such low voltages, saturation of the active transistors can occur and slow the rise time capability of the write driver circuit and thereby limit the data rate at which the circuit can operate. No means, other than employing a high supply voltage, is provided to prevent large differential inductive voltage transients from causing saturation of the then active transistor of the lower pair.
Disk storage systems are now being proposed that require low, single (grounded) supply voltages without saturating the active transistors, low power consumption, high transition (i.e., transfer) rates, and means for ensuring that no active transistors will saturate even when large differential inductive voltage transients exceed the supply voltage. This combination of attributes is not achievable with the above-cited and other prior art configurations known to applicants.